US2583785A - Biasing protective system for high-power oscillators - Google Patents

Description

Jan. '29, 1952 2,583,785

P. B. MAGEE BIASING PROTECTIVE SYSTEM FOR HIGH-POWER OSCILLATORS Filegl Nov. 26, 1949 0 MAM IN PUT '15 VOLTS 6O CYCLE.

Inventor: Paul B. Magee,

Hus Attorneydifierent frequencies.

Patented Jan. 29, 1952 UNITED STATES PATENT OFFICE BIASING PROTECTIVE SYSTEM FOR HIGH-POWER OSCILLATORS Paul B. Magee, Buffalo, N. Y., assignor to General Electric Company, a corporation of New York My invention relates to a system for controlling and protecting a self-biased electron discharge oscillator operating at high power levels.

In induction heating systems, the same high power oscillator is often employed with any one of a number of different work circuits, either for different heating applications or for operation at For example, a number of different resonant tank circuits are often ar ranged to be selectively switched into circuit with a high power oscillator tube. Before switching tank circuits, it is the usual practice to disconnect the high voltage anode suppiy source. In fact this has heretofore been important when switching in a conventional oscillator circuit of the Hartley type having grid-self-bias. Otherwise, disconnection of the tank circuit. results in loss of grid bias and excessive anode current will very quickly damage or destroy the oscillator tube.

In a high power system of this prior art type, the anode switching must be done at high power levels, requiring a heavy contactor in the anode circuit, and unless interlocks are provided, there is still the danger that the tank circuit may be disconnected, unintentionally or otherwise, while the anode voltage is still applied.

In accordance with my invention, the need for anode switching is completely eliminated and the oscillator tube is positively and automatically protected against damage while switching work circuits. Yery briefly, this is accomplished by employing an auxiliary bia rectifier, with its negative output terminal connected through a suitable coupling network to the grid circuit of the power oscillator tube. During normal operation, the bias voltage thus produced is effectively short-circuited to ground through a portion of the tank circuit itself, but when the tank circuit is removed from the oscillator, the oscillator tube s cut off by this negative bias to avoid damage or destruction.

It is, therefore, an object of my invention to provide a simple and reliable protective system for a high power oscillator having a disconnectable tank circuit.

Another object of my invention is to provide an improved oscillator system of this general type in which disconnection of the tank circuit automatically reduces the anode current to a value below the maximum rating of the oscillator tube.

A further object of my invention is to provide the use of switching apparatus operating at high power levels for controlling the output power of a high power oscillator, for use in inductive heating applications and the like.

The novel features which I believe to be characteristic of my invention, are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, may best be understood by reference to the following description taken in connection with the single figure of the accompanying drawing which is a circuit diagram of an oscillator system embodying my invention.

Referring to the drawing, I have shown a schematic electric circuit diagram of an induction heating system comprising a suitable full wave rectifier power supply I, a conventional Hartley high-power, high-frequency, oscillator 2 and an auxiliary bias rectifier 3. This auxiliary bias rectifier 3 and its function in the circuit will later be described more fully. Oscillator 2 also in eludes a parallel resonant heating circuit 4 which is shown as a separate unit having a plurality of plug-and- jack connectors 5, 6 and I which enable it to bereadily disconnected from and reconnected to the remaining oscillator circuits. Tank circuit 4 includes inductor {i and capacitor 9. Depending upon its application, tank circuit 4 may take on various physical shapes and sizes.

The power supply I may be any suitable conventional rectifier supply which supplies anode potential to a suitable electron discharge device 10, which is preferably of the high vacuum type, having an anode l I, control grid l2 and cathode 13. The anode potential is filtered by a network comprising resistor bank l4, capacitor i5, and inductor l6, and then applied to the anode ll of the electron discharge device Ill.

Electron dischage device I0 is included in a conventional Hartley oscillator circuit which supplies the high frequency heating current to the tank circuit 4. Thus, anode II is connected to the upper terminal of tank circuit 4 through a blocking capacitor l1 and connector 5. Cathode I3 is connected to ground through the secondary of a filament transformer l8 and bypass capacitors l9. An intermediate tap point 20 on induc tor 8 is also connected tov ground through connector 6. Grid I2 is connected to the lower terminal of tank circuit 4 through a series circuit including a resistor 2|, a grid capacitor 22 and grid leak 23 in parallel, and connector 1. Oscillator 2 is of the self-biasing type, grid bia being developed across grid capacitor 22 and grid leak 23 in response to grid current in electron discharge device l3. The operation of such an oscilator is well understood by those skilled in the ar The rectifier 3. which is of conventional fullwave type, cause a negative potential with respect to ground to appear at the junction of a pair of load resistors 24 and 25. This negative voltage bias is transmitted through a coupling network consisting of a resistor 21 and choke 26 to the junction of'capacitor 22 and resistor 23. When the tank circuit l is pluggedinto the circuit,

, the relatively low D.-C. impedance of inductors said grid circuit, and bein normallygloaded by said resistance, said bias potential being suflicient to prevent damage to said device from excessive anode current when said inductance is disconnected from said grid circuit while anode potential is applied. 7

, 2-. An oscillator protective system comprising an electron discharge device having an anode, cathode, and control grid, a source of anode operating potential connected between said anode and cathode, a parallel-resonant load circuit into replace it with a more suitable one or perhaps accidentally, the negative voltage from rectifier 3 is no longer short-circuited to ground by inductor 8, and this negative voltage is transmitted, through resistors 23 and 2! to the control electrode l2 of electron discharge device it. This negative voltage or bias may be of such a value as rto completely cut off any anode current in the electron discharge device it, or it may *be adjusted, by proper selection of the values of resistors 2G and 25, to a value which will merely limit the current in the electron discharge device it to a safe level. Without such a bias on the control electrode of electron discharge device ill, the removal of the; tank circuit 3 would cause the control electrode, to be at essentially .zero poten tial, and the high current in electron discharge device l3 would be great enough to bring about the destruction of the cathode. However, in the system shown, it is to be noted that removal of the tank circuitv cutsoff the highpower oscillator electron discharge, device without any additional switching in the anode supply circuit.

While I have shown a particular circuit designed to operate in accordance with my inven tion, it'will be understood that this is by way of illustration of the principles involved and that thoseskilled in the art. may make many modifications in the circuit arrangement'and mode of operation. For example, the oscillator 2 may be a push-pull type. Therefore, I contemplate by the appended claims to. cover any such modifications as fall within the truev spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1 An oscillator protective system comprising an electron discharge device having anode and grid circuits, a source of, operating potential for said anode circuit, a resonant tank circuit having an inductive branch of low resistance, said the portion offsai'd inductive branch included in eluding an inductance, a connection from one terminal of said circuit to said anode, a connection from the other terminal of said circuit to said grid through a detachable connector and through a grid self-bias network in the order named, a connection from an intermediate tap point onsaid inductance to said cathode through a detachable connector, and an auxiliary source of grid bias potential having its positive terminal connected to said cathode and its negative terminal connected to said grid through said network, said source being normally substantially short-circuited by theportion of said inductance included. between said connectors, the potential of said auxiliary source beingsufiicient to limit the anode current in said device to a non-destructive value when either of said connectors is dis connected while anode potential is applied.

'3. A protective system for a high-power oscillator of the Hartley type, comprising an electron discharge device having grid and anodecircuits, means for impressing anode operating potential on said anode circuit, a resonant tank circuit including an inductance regeneratively coupling said grid and anode circuits for generation of oscillations, a grid. self-bias network in said grid circuit for establishing a normal anode operating current, said. resonant circuit being mechanically arranged for connection and disconnection as a. unit to, and from the remainder of said circuits, whereby said device loses its self bias and tends to draw destructive anode current if said resonant circuit is disconnected while anode potential is applied, and an auxiliary protective source of negative bias potential connected in said, grid circuit, said source having a relatively high impedance and being normally loaded by the relatively low-impedance portion of said inductance in circuit therewith, the voltage of said source being sufficient, upon disconnection of said resonant circuit, to limit said anode current to a safe value insufiicient to damage said device.

PAUL B. MAGEE.

REFERENCES CITED Number Name 7 Date 1,938,160 Wallace Dec. 5, 1933 2,503,073 Schreiner Apr. '4, i950

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